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Probing Question: What is a supersolid?

Joseph Gyekis

May 11, 2005

Probing Question: What is a supersolid?

"Imagine you have an orchestra together, but everyone is playing their own tune, until they begin to follow a conductor. In a normal solid, every atom has its own behavior until very close to absolute zero. Then quantum mechanics takes over and dictates everyone to play the same tune."

Oscillator

That's physics professor Moses Chan's musical metaphor for his discovery that atoms in a solid can condense into what he likes to call "one giant atom," a new phase of matter called a supersolid. Together with post-doctoral associate Eun-Seong Kim, Chan found that when a particular isotope of helium gas has frozen into a crystal at a fraction of a degree above absolute zero, part of it exhibits a property only seen before in superfluids: no friction.

To understand frictionless flow, says Chan, think of a bunch of kids sitting on a spinning merry-go-round. Normally, the more kids on the merry-go-round, the harder it is to stop the movement and reverse its direction. Chan and Kim set up an oscillator that spins back and forth like a merry-go-round shifting direction. They found that helium crystals in a normal solid state behaved as expected, with each additional bit of crystal adding to the mass of the "merry-go-round" and increasing the resistance.

However when those same crystals are frozen below 0.2 degrees Kelvin, something unexpected happens: one percent of the solid helium begins to flow without resistance. "It's as if a portion of the kids on that merry-go-round are sitting on perfectly smooth ball bearings, unaffected by the merry-go-round sliding back and forth underneath them," explains Chan. This allows the crystal to oscillate faster, as if the crystal has suddenly become lighter—or the kids have lost weight in mid-spin. Chan and Kim knew that the matter had not been lost because the missing mass re-materialized with the slightest rise in temperature, and the oscillation slowed back down to normal.

Although the existence of supersolids was predicted decades ago, prior attempts to find evidence for them had come up empty. "One of the reasons why this phenomenon has not been seen before is that no other experimental group has oscillated the solid helium as gently as we have," Chan explains. "With harder oscillations, the superflow effect will go away."

Kim and Chan's result forces theoretical physicists to rethink how to distinguish solids from liquids when considering quantum effects.

In quantum terms, Chan points out, the behavior of any atom can be described both as a particle and as a wave-packet. An individual wave-packet increases in size as it is cooled, especially when the thermometer drops close to absolute zero. Where at higher temperatures atoms are normally locked in a grid, like rows of people sitting in an auditorium, near zero the wave-packets expand and overlap with their neighbors.

In classical physics, objects cannot share the same space. "If I run into you, there will be a collision and the motion will stop," Chan says. "But in quantum mechanics, we become one thing."

When the supercooled helium atoms expanded out into one another, he continues, they lost their individuality and became one giant atom. It's as if that theater audience became a single, room-sized person.

Moses Chan, Ph.D., is associate director of the Center for Nanoscale Science and Evan Pugh professor of physics at Penn State. Eun-Seong Kim, Ph.D., is a post-doctoral associate in physics. Their work was published in the journal Nature in January 2004.